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Fabricating PFPE Membranes for Microfluidic Valves and Pumps

Thursday, 30 April 2009

This process contributes to development of “laboratory-on-a-chip” devices.

A process has been developed for fabricating
membranes of a perfluoropolyether
(PFPE) and integrating them into
valves and pumps in “laboratory-on-a-chip”
microfluidic devices. Membranes
of poly(tetrafluoroethylene) [PTFE]
and poly(dimethylsilane) [PDMS] have
been considered for this purpose and
found wanting. By making it possible to
use PFPE instead of PTFE or PDMS, the
present process expands the array of
options for further development of
microfluidic devices for diverse applications
that could include detection of
biochemicals of interest, detection of
toxins and biowarfare agents, synthesis
and analysis of proteins, medical diagnosis,
and synthesis of fuels.

To be most useful, a membrane material
for a microfluidic valve or pump
should be a chemically inert elastomer.
PTFE is highly chemically inert and is a
thermoplastic and, therefore, subject to
cold flow and creep. Also, procedures for
fabricating PTFE membranes are excessively
complex. PDMS is an elastomer
that has been used in prior microfluidic
devices but, undesirably, reacts chemically
with some liquids (acetonitrile, acids,
and fuels) that might be required to be
handled by microfluidic devices in some
applications. On the other hand, the
PFPE in question has elastomeric properties
similar to those of PDMS and a
degree of chemical inertness similar to
that of PTFE.

The specific membrane material to
which the present process applies is a
commercially available, ultraviolet-curable
PFPE. A microfluidic device of the
type to which the process applies consists
mainly of this PFPE sandwiched
between two plates of a highly chemically
resistant, low-thermal-expansion
borosilicate glass manufactured by the
float method. Heretofore, there have
been two obstacles to fabrication of
microfluidic devices from this combination
of materials: (1) The lack of chemical
reactivity between the PFPE and the
glass makes it impossible to form a lasting
bond between them; and (2) such
conventional membrane-fabrication
techniques as spin coating yield membranes
that are not sufficiently flat and
not sufficiently resistant to curling upon
themselves. The present process overcomes
these obstacles.

The process consists mainly of the following
steps:

A fluorocarbon-based polymer is
formed on the glass plates by means
of a plasma deposition subprocess.

The polymer is patterned by use of a
photoresist and conventional photo-lithography.

The polymer is removed in the pattern
by use of an O2/Ar plasma.

The remaining polymer surface areas
are cleaned and modified by use of a
low-energy O2 plasma.

The glass plates are spin-coated with a
lift-off material, which is then cured
by heating to a temperature of 150 °C
for 5 minutes.

The liquid (uncured) PFPE material
is pressed between the two lift-off-layer-coated glass plates, along with
250-μm-thick shims to define the
desired thickness of the PFPE membrane.

The liquid PFPE is cured to a solid by
exposure to ultraviolet light for 5
minutes.

The PFPE membrane is released
from the glass plates by submersion
in a developer solution and/or acetone.

The glass plates and the PFPE membrane
are cleaned and activated for
bonding by exposure to an O2 plasma.

The glass plates and the membrane
are aligned and sandwiched together
at a temperature ≤100 °C and a pressure
of 3 bar (0.3 MPa) for one hour.
This combination of pressure and
temperature is sufficient to cause a
chemical reaction that results in
bonding of the PFPE membrane to
the polymer coats on the glass plates.

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